LA Study Tour with Jack
Venice: Rising Water, Sinking Land
by Ben Webb

Background

Venice (Google Maps | MSN Maps) is built on a system of 118 small islands interspersed with a network of 160 canals (Pearsons, 2009).  It is sited in the northeast of the Adriatic Sea in the Venetian Lagoon.  Built on millions of sunken wood pilings, which will not rot in the anaerobic environment below the water, (BBC, 2003) the city is vulnerable to sea and groundwater level change.  If the water level is too low the supporting pylons will rot; too high and the brick and stone buildings begin to crumble from contact with the salt water.  Fortunately, Venice does not have to worry about the former.

Gondola ride in San Marco Plaza

Figure 1 - A gondola ride in San Marco Plaza 
during an Acqua Alta event.
(Source: pro.gra.ms srl postcard, date unknown)

Acqua Alta (high water) events are both annoying and damaging.  The salty lagoon water is polluted by mainland farming practices and untreated sewage; only recently has Venice and the surrounding countryside begun to treat it sewage before it is dumped into the lagoon.  The annoyance of the pollution, a mess that needs to be cleaned up after the water recedes, pales in comparison to the additional damage caused by the salt-water corrosion of the brick load-bearing walls.

“Instead of merely washing against the impermeable marble that makes up the city’s foundations, high waters are splashing with increasing frequency against the soft, permeable bricks that sit above the foundations.  Saltwater from the Adriatic soaks into this brick, inching ever higher into the walls and creeping into the interiors, destroying frescos and other irreplaceable relics.  Unless they have been restored with new, waterproof brick, many of these buildings crumble imperceptibly.” (PBS, 2002)

Water damage - Crumbling brick

Figure 2 - Saltwater damage
to brick and stucco facade.
(Source: B. Webb, 2009)

Map showing flooding in Venice

Figure 3 – Flooding in Venice.
(Source: City of Venice, date unknown)

Venice’s elevation averages 100 cm above the mareographic zero at "Punta della Salute" and is approximately 80 cm above the mareographic zero at its lowest point, Piazza San Marco (City of Venice, 2009).  Historically, the city has been periodically inundated with water and until relatively recently, these acqua alta (high water) events were few and far between.  A normal high tide in the lagoon is about +80 cm; an acqua alta is declared when the high tide is above 110 (City of Venice, 2009).  Figure 3 shows the area of the city covered by water at 100 cm (red), 120 cm (dark blue), and 140 cm (light blue); these areas represent 3.56%, 35.18%, and 90.19% of the city respectively (see table 1).  Figure 4 shows a person standing in Piazza San Marco at various acqua alta levels.

High Water Level 
Venice Flooded
Venice Flooded
(above mareographic zero)
(Ha)
(%)
Up to 90 cm
1.1135
0.29%
100 cm
13.8763
3.56%
110 cm
45.8017
11.74%
120 cm
137.3544
35.18%
130 cm
268.4702
68.75%
140 cm
352.211
90.19%
150 cm
376.1634
96.33%
160 cm
387.6498
99.27%
170 cm
389.4623
99.74%
180 cm
389.9061
99.86%
190 cm
390.4311
100.00%

Table 1 – Water level heights above mareographic zero with corresponding area and percent of Venice flooded.
(After: Citta di Venezia, “La perentuale di allagamento”, 2009)

Diagram of Flood Levels in St. Marks Square

Figure 4 – Diagram Showing Various Flood Levels in St. Marks Square.
(Source: Citta  di Venezia, 1968)

Traditionally, acqua alta, were due to unusually high tides caused by strong prevailing winds, storm surges, and severe inland rain events (PBS, 2002).  A particularly bad acqua alta in 1966 combined with an increasing frequency (figure 5) of acqua alta caused many Venetians to wonder if their city was sinking.  After much research, the conclusion is that the city is sinking in a process called subsidence and sea levels are rising in a process called eustasy. 

Number of Acqua Alta events per decade (1920-1999)

Figure 5 - Frequency of Acqua Alta events ≥ +100 cm
by decade 1920 - 2000, Venice.
(Source: City of Venice, date unknown)

Piazza San Marco during an Alta Acqua

Figure 6 - San Marco Plaza during an Acqua Alta.
(Source: Davide Toffanin, City of Venice, 2008)

Historically, Venice is most likely to be affected by acqua alta during the autumn and winter (the wettest part of the year is between October and January).  During the wet season, sections of Venice are underwater on a daily basis and a majority of the city is flooded half a dozen times a year.  However, due to subsidence and eustasy, this is happening more often now (figure 5).  A century ago, St. Marks Square (figure 6), one of the lowest points in the city, flooded about 9 times a year; now it is inundated with water approximately 100 times per year (PBS, 2002). 

Venice is Sinking

Figure 7 - Venice is Sinking.
(Source: B. Webb, 2009)

Subsidence in Venice is caused by both natural and manmade factors.  In the 1950s a deep-water channel was dug along the edge of the Venetian lagoon to allow oil tankers to dock at a new refinery.   Additionally, land-based industry, near Venice, significantly lowered the ground water level in a large aquifer deep below Venice through overuse of artesian wells.  These two manmade events caused subsoil compaction and the rapid sinking of Venice.  By the 1970s, Venice had sunk almost 20 centimeters (BBC, 2003).  Fortunately when the Venetians realized what was happening in the 1980s, the artesian wells were closed off and the drilling of new wells banned.  This helped to slow the sinking.  However, Venice is still subsiding at a much slower rate due to a natural process - tectonic plate subduction.  Venice lies on a plate in the Adriatic that is being forced downwards and underneath another Italian plate.  This has the effect of lowering Venice at a rate of about 1 mm per year (New Scientist, 2003).

Nowadays, the rate the city is sinking is inconsequential compared with the expected rate of global sea level rise due to climate change.  Global climate change threatens to put Venice on the frontlines in a new war against rising sea level – involving many coastal cities worldwide.

 

Response to Rising Sea Level in Venice

A number of solutions have been proposed that would help forestall the watery grave that could be Venice’s future.  There is doubt that Venice will be able to stave off forever a fate similar to that of the fabled city of Atlantis.  Short term, cheap, simple solutions such as raising the sidewalks and plazas in the lowest areas, raising the entrances to buildings, and moving up off of the first floor have been proposed and largely implemented in the hardest hit areas. 

Temporary Pedestrian Routes during Acqua Alta

Figure 8 - Temporary Pedestrian Routes during an
Acqua Alta. (Click to enlarge).  Red lines indicate
temporary walkways and Green lines indicate
permanent walkways higher than +120cm.
(Source: City of Venice, Alternative Routes, 2008)

Between September 15 and April 30, the Venice City Council implements the ‘Pedestrian Mobility Plan’ with temporary raised walkways managed and erected by Vesta s.p.a., a multiple-municipality public works company.  The plan involves erecting raised walkways along the lowest sections of street which provide access to the main public buildings in the city centre and to the public transit terminals with connections to the mainland, islands, and coast.  The red and green routes shown on the map (figure 8) provide the minimum use of walkways to ensure dry mobility during acqua alta events of up to about +120cm.  Above this level, some routes may be impassable as certain sections may be flooded or the walkways may be submerged with the risk of floating (Citta di Venezia, Alternative Pedestrian Routes, 2008).

More radical solutions involve demolishing old buildings and rebuilding on higher foundations (New Scientist, 2003).  However all of these solutions are temporary at best and only work until the canal bridges become too low for boats to pass underneath and buildings are permanently damaged.

Location of MOSE Gates

Figure 9 - Location of MOSE Gates.
(Source: NOVA, 2002)

Predictably, large-scale engineering feats have also been proposed to solve the problem.  The most widely known proposal is called the MOSE (Modulo Sperimentale Elettromeccanico) project.  Proposed since the 1970s, this project is basically a series of floodgates that would stretch across the three openings that connect the Venetian Lagoon with the Adriatic Sea (figure 9).  When tides are low, the floodgates would lie flat on the bottom of the channels.  When an unusually high tide is detected, the gates would be activated by filling with air and rising up into place, creating a barrier to the Adriatic (figure 10).  This proposed barrier would hold back the incoming tide up to a level of 2 meters above the water level of the lagoon (New Scientist, 2003).

The MOSE Gates in Action The MOSE Gates in Action
Figure 10 - The MOSE Gates in action.
(Source: NOVA, 2002)

The MOSE project is not without its problems or critics.  However, the government announced plans to begin work on the project in 2001.  It was expected to cost between $2 and $3 billion and would have been completed by 2010.  Somewhat surprisingly, the project was put on hold as the Venice City Council voted to reconsider its support and reevaluate the project in 2002 (New Scientist, 2003).  Some of the concerns were that the gates would be obsolete in a few years time due to increasing rates of sea level rise; that the lagoon would be closed for too long – disrupting the natural ebb and flow of tides, resulting in increased pollution concentrations and environmental damage.  In 2003, the project was relaunched by Silvio Berlusconi, Italy’s Prime Minister.  It is now expected to by complete by 2012 at a cost of $7 billion (NPR, 2008). 

 

Venice - the canary in the coal mine

Regardless of what the Venetians do, the problems of subsidence and eustasy will continue long into the future.  What the future holds is uncertain; but these problems are not new and not unique to Venice.  As global climate change continues to raise sea water levels throughout the world, many cities and countries will look to Venice and learn from their experience.  Rising (or falling) sea levels will reshape the world’s coastlines and can affect some of the most densely populated areas on earth. 

“Eight to ten million people live within one meter of high tide in each of the unprotected river deltas of Bangladesh, Egypt and Vietnam.  Half a million people live in archipelagos and coral atoll nations that lie almost entirely within three meters of sea level, such as the Maldives, the Marshall Islands, Tuvalu, Kiribati, and Tokelau.  Other archipelagos and island nations in the Pacific, Indian Ocean, and Caribbean could lose much of their beaches and arable lands, which would cause severe economic and social disruption.” (U.N. Intergovernmental Panel on Climate Change, 1990) 

Graph showing the change in average sea level from 1993 to 2005

Figure 11 - Graph showing the change in average
sea level from 1993 to 2005. On average, sea level
rose 3mm per year - half was attributed to melting
ice and half to the thermal expansion of water.
(Source: Remer, 2008)

There are many reasons for sea levels to rise, a few discussed previously are unique to Venetians; a few more universal causes of sea lever rise are: “thermal expansion [of water] – the tendency of warm water to take up more space than cooler water… [the] addition of water, for instance from melting glaciers…. [and] changes in salinity; fresh water is less dense than salt water and therefore takes up slightly more space than an equal mass of salt water.” (Remer, 2008)

In order to safely protect human life and property, there are three broad categories of strategic response to rising sea levels: retreat, accommodation, and protection (see figure 12).  Retreat entails no effort to save the land from the sea; the coastal zone is abandoned and theoretically ecosystems shift landward.  Accommodation implies that “people continue to use the land at risk but do not attempt to prevent the land from being flooded.” (U.N., 1990).   Protection involves engineered solutions such as sea walls, levees, and dikes, as well as, ecological landscape solutions (green infrastructure), such as restoring dunes, natural levees, and vegetation in the coastal flood plain.

“The appropriate mechanism for implementation depends on the particular response.  Assuming that land for settlement is available; retreat can be implemented through anticipatory land use regulations, building codes, or economic incentives.  Accommodation may evolve without governmental action, but could be assisted by strengthening flood preparation and flood insurance programs.  Protection can be implemented by the authorities currently responsible for water resources and coastal protection.” (U.N., 1990)

Strategies for adaptation to rising sea levels

Figure 12 - Diagram showing a few strategies for adaptation
to rising sea levels.
(Source: U.N. Intergovernmental Panel on Climate Change, 1990)

Despite the reason, be it subsidence, eustasy, thermal expansion of water, addition of water from melting glaciers, or changes in salinity, cities and countries worldwide are threatened by rising sea levels and global climate change.  As these problems continue into future, many places will face the same issues that Venice faces today.  Rising sea levels will reshape the world’s coastlines and will affect some of the most densely populated areas on earth.  We must respond appropriately to the threat today, with the previously mentioned strategies, to avoid mass loss of life and severe economic and social disruption. 

 

 

Sources:

BBC News. “Venice’s 1,500- year battle with the waves”. 2/17/2003. BBC News : Europe. http://news.bbc.co.uk/2/hi/europe/3069305.stm (2/25/09)

Citta de Venezia. “La perentuale di allagamento”.  2009  City of Venice. http://www.comune.venezia.it/flex/cm/pages/ServeBLOB.php/L/IT/IDPagina/1502 (3/30/09)

Citta de Venezia. “La perentuale di allagamento”.  1968.  City of Venice. http://www.comune.venezia.it/flex/images/D.ebba1eb0431edc565e5a/stivaloni4_2.jpg (3/30/09)

Citta di Venezia - Instituzione Centro Previsioni e Segnalazioni Maree. “High Water – Alternative Pedestrian Routes”. City of Venice, 2008.

Citta di Venezia. “Servizi di informazione e di comunicazione”. City of Venice. http://www.comune.venezia.it/flex/cm/pages/ServeBLOB.php/L/IT/IDPagina/2420 (3/30/09)

City of Venice. “F.A.Q. High Tide in Venice”. City of Venice. http://www.comune.venezia.it/flex/cm/pages/ServeBLOB.php/L/EN/IDPagina/22795 (2/24/09)

NPR. “MOSE Project Aims to Part Venice Floods”. 1/7/2008. Morning Edition : NPR. http://www.npr.org/templates/story/story.php?storyId=17855145 (2/25/09)

Pearsons Australia. “Venice, Sinking City.” 2009. Infobase Atlas. http://www.hi.com.au/resource/rfactsa.asp?kla=13&subtopicid=3599 (2/25/09)

PBS. “Sinking City of Venice.” 2002. NOVA. http://www.pbs.org/wgbh/nova/venice/solutions.html (2/25/09)

PBS. “Sinking City of Venice.” 2002. NOVA. http://www.pbs.org/wgbh/nova/venice/ (2/24/09)

PBS. “Sinking City of Venice.” 2002. NOVA. http://www.pbs.org/wgbh/nova/transcripts/2914_venice.html (2/24/09)

Remer, Lorraine. “The Rising Sea Level”. Sept 22, 2008. NASA Earth Observatory. http://earthobservatory.nasa.gov/IOTD/view.php?id=6638 (4/1/09)

U.N. - Intergovernmental Panel on Climate Change. “Report of the Coastal Zone Management Subgroup: Strategies for Adaptation to Sea Level Rise”. November, 1990.

“Venice Dragged Down by Force of Nature”. New Scientist. Issue 2406, p. 20. August 2, 2003